1. Field of the Invention
This invention relates to zoom lenses, and more particularly to zoom lenses having the focusing provision in part of lens components which remain stationary during zooming.
2. Description of the Prior Art
In conventional zoom lens having a variator preceded by a focusing component, the characteristics that the once-established in-focus condition is not broken by any subsequent zooming operation, and that, in any zooming position, focusing is effected by a predetermined amount of forward movement for a certain object distance have been enjoyed.
As compared with this, other focusing methods by moving one of the components that follows the variator have to differentiate the amount of forward movement of the focusing component depending upon the zooming position despite a constant object distance. Therefore the focusing control or cam mechanism tended to be complicated in structure.
For this reason, many zoom lenses have the focusing provision made at a component in front of the variator, or, for example, the first component counting from front.
This focusing arrangement is, however, accompanied with the following disadvantages:
(1) Since the diameter of the first component is generally large, and its weight is also heavy, the operativity when in focusing becomes bad.
(2) As the first component moves forward to effect focusing down to shorter object distances, the total length of the entire lens system becomes longer, and the weight balance gets worse. (3) When the first component is driven to move by automatic focusing mechanism, the driving system must be of high torque, making it difficult to operate the focusing system quickly with high accuracy.
To avoid these, a proposal has been made that the front or first component is divided into two parts of which the rear part only is moved to effect focusing, namely, the so-called inner focusing method, as, for example, disclosed in Japanese Laid-Open patent applications Sho Nos. 52-109952, 55-57815 and 55-117119 and U.S. Pat. Nos. 3,598,476, 4,110,006 and 4,099,845.
In those lens system employing the inner focusing method, from the standpoint of aberration correction, the first component is made constructed in a considerably complicated form, and has a considerably heavy weight.
In Japanese Pat. Publication Sho No. 52-41068, it is proposed that the first lens component is constructed with two lens groups of negative and positive powers, and the rear or positive lens group is made movable for focusing.
This focusing method when applied to the positive lead type of zoom lenses where the front lens group is of positive refractive power, produces the following drawbacks:
(4) Since the first component is of positive refractive power, letting f.sub.I denote the focal length of the first component, and f.sub.IA and f.sub.IB the focal lengths of the front and rear lens groups of negative and positive powers respectively, we have the following approximate equation: EQU f.sub.I =f.sub.Ia =f.sub.IB
At this time, f.sub.I &gt;f.sub.IA and .vertline.f.sub.IB .vertline.&gt;f.sub.IA are given. Therefore, the focal length f.sub.IB of the focusing lens group becomes small, making it difficult to assure good correction of aberrations throughout the focusing range. Also from the aspect of production, high precision techniques are required.
(5) In order that the rear lens group of positive power admits of the diverged light pencil by the front or negative lens group, the diameter of the rear lens group is necessarily increased.